Abstract. This article focuses on a selection of satellite infra-red IASI observations and their simulation in the global Numerical Weather Prediction (NWP) system ARPEGE (Action de Recherche Petite Echelle Grande Echelle), using the sophisticated radiative transfer model RTTOV-CLD which takes into account the cloud multi-layers and the cloud scattering from atmospheric profiles and cloudy microphysical parameters (liquid water content, ice content and cloud fraction). The aim of this work is to select homogeneous scenes by using information of the collocated Advanced Very High Resolution Radiometer (AVHRR) pixels inside each IASI field of view and to retain the most favourable cases for the assimilation of IASI infrared radiances. Two methods to select homogeneous scenes using homogeneity criteria already proposed en the literature were employed; criteria derived from Martinet et al. (2013) for cloudy sky selection in the French mesoscale model AROME (Applications of Research to Operations at MEsoscale), and the criteria from Eresmaa (2014) for clear sky selection in the global model IFS (Integrated Forecasting System). An intercomparison between these methods reveals considerable differences, either in the method to compute the criteria or in the statistical results. From this comparison a revised method is proposed that is a compromise between the different tested methods, using the two infrared AVHRR channels to define the homogeneity criteria in the brightness temperature space. This revised method has a positive impact on the observation statistics minus the 15 simulation statistics, while retaining 36 % observations for the assimilation. It was then tested in the NWP system ARPEGE and tested for the clear-sky assimilation. These criteria were added to the current data selection based on the Mc Nally and Watts (2003) cloud detection. It appears that the impact on analyses and forecast is rather neutral.

How to cite: Farouk, I., Fourrie, N., and Guidard, V.: Homogeneity criteria within IASI pixels for the preparation of an
all-sky assimilation, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2018-288, in review, 2018.

A selection for homogeneous scenes for the all-sky infrared radiance assimilation is proposed by using information of the collocated imager pixels inside each infrared observation. A revised method for the selection, which is a compromise between two methods to select homogeneous scenes using homogeneity criteria already proposed en the literature has a positive impact on the observation minus the simulation statistic. It has been tested in a numerical weather prediction model for clear sky.

A selection for homogeneous scenes for the all-sky infrared radiance assimilation is proposed by...